It is usual in terrestrial (onshore) seismic techniques, and in particular in vertical seismic techniques, to use seismic sources of the air gun type which are often placed in shallow boreholes, typically of the order of 20 to 30 m in depth. Seismic energy from the source that is propagated through the earth is then detected and recorded by receivers positioned some distance away from the source. In the case of vertical seismic techniques, the receivers are positioned in a borehole such that they may be up to several thousand meters distant from the source with the objective of investigating the structure and properties of the subsurface.
It has already been proposed in order to have substantial seismic signals at the source to use a plurality of seismic sources in the neighbourhood of each other.
A problem that occurs with such an arrangement of sources is the accurate synchronisation of the individual source outputs.
When a source is deployed in a shallow borehole, it is not usual to be able to accurately determine its position. The extent of the uncertainty may be of the order of 1 or 2 meters horizontally and 1 meter vertically.
Further, the region of earth immediately local to each source can have differing properties from source to source position and from time to time such that the propagation of seismic energy through them also varies from source to source and time to time.
Such factors prevent the use of existing techniques usually used to control the synchronisation of the shots of the sources, since these require precise knowledge of the source locations, and assume uniformity in the region of the earth local to the sources.
The invention proposes a solution that permits a solution to these problems.